In fuel cell power plants, it is necessary to cool the fuel cell, either by means of porous water transport plates which allow diffusion of both water and reactant gases therethrough, or by means of coolant plates, or both. In fuel cell power plants employing proton exchange membrane (PEM) fuel cells, compactness and minimal parasitic power suggest the use of water transport plates.
It is also known to use a separator to separate air and frothy water from the main coolant supply prior to returning the coolant to a pump, in order to avoid pump cavitation. One form is illustrated in FIG. 1. Therein, the primary coolant pump 11 draws water through a separator 12 from a conduit 13 connected to the coolant outlet 14 of a fuel cell stack assembly 15. The coolant inlet 18 is connected by a conduit 19 to a coolant outlet 20 of a liquid accumulator 21, which has a conventional vent 29.
The outflow of the coolant pump 11 is provided by conduit 23 to the accumulator 21. Pump outflow is also provided over a conduit 24 through a pressure reducing orifice 25 to a demineralizer 26, so that some fraction of the circulating water is demineralized (has ions removed therefrom), continuously. The coolant water outflow of the pump 11 is also provided over a conduit 30 to the primary inlet 31 of an eductor (ejector) 32. The secondary inlet 33 of the eductor is connected to the separator 12, and serves to suction off frothy water and air from the separator 12. The outlet of the eductor 37 is connected by a conduit 38 to the accumulator 21.
Problems with this prior art system arise due to the difficulty of matching the water flow requirements of the pump 11 with the inlet pressure requirements of the eductor 32. It is essential that the eductor inlet pressure be properly selected to provide the desired function of clearing the separator 12. It is also essential that the pump 11 provide the proper water flow at a desired pressure at the water outlet 14 of the fuel cell stack 15.